1. Field of the Invention
This invention relates to a radiation image storage panel. This invention particularly relates to a radiation image storage panel provided with a stimulable phosphor layer, which is capable of emitting light when being exposed to stimulating rays.
2. Description of the Related Art
Radiation image recording and reproducing systems comprising radiation image recording apparatuses, radiation image read-out apparatuses, and the like, in which stimulable phosphors are utilized, have heretofore been known as computed radiography (CR) systems. With the CR systems, a radiation image of an object, such as a human body, is recorded as a latent image on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored on the stimulable phosphor sheet during exposure of the stimulable phosphor sheet to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. In this manner, the image signal representing the radiation image of the object is acquired.
As a recording medium to be used in the radiation image recording and reproducing systems described above, a radiation image storage panel provided with a stimulable phosphor layer, which has been formed with a process for coating particles of a stimulable phosphor onto a substrate, or a radiation image storage panel provided with a stimulable phosphor layer comprising pillar-shaped crystals of a stimulable phosphor, which stimulable phosphor layer has been formed on a substrate with a vacuum evaporation process, is utilized. It has heretofore been known that a distribution of a light intensity of the emitted light, which is emitted from the stimulable phosphor layer of the radiation image storage panel when the stimulating rays are irradiated to the radiation image storage panel, with respect to the light radiating angle, at which the emitted light is radiated out from the stimulable phosphor layer, is biased toward a direction, which is normal to the surface of the stimulable phosphor layer. (The distribution of the light intensity of the emitted light, which is emitted from the stimulable phosphor layer of the radiation image storage panel when the stimulating rays are irradiated to the radiation image storage panel, with respect to the light radiating angle, at which the emitted light is radiated out from the stimulable phosphor layer, will herein below be referred to as the light emission angle distribution.) FIG. 11 is an explanatory view showing a cos θ distribution. Specifically, in FIG. 11, P1 represents the so-called cos θ distribution, wherein the relationship between a light intensity Kθ of the emitted light, which is radiated out from a stimulable phosphor layer 1 toward a direction H normal to the surface of the stimulable phosphor layer 1, and a light intensity kθ of the emitted light, which is radiated out from the stimulable phosphor layer 1 toward a direction making an angle θ (i.e., at a light radiating angle θ) with respect to the direction H normal to the surface of the stimulable phosphor layer 1, is represented by the formula of kθ=K0×cos θ. As illustrated in FIG. 11, the emitted light, which is radiated out from the stimulable phosphor layer 1, has a light emission angle distribution (represented by P1′ in FIG. 11) that is compressed in the direction, which is normal to the direction H normal to the surface of the stimulable phosphor layer 1, and into an oblate distribution, which is flatter than the cos θ distribution.
The surface of the stimulable phosphor layer has converging effects, and therefore the emitted light, which is radiated out from the stimulable phosphor layer, has the light emission angle distribution described above. Specifically, in the cases of the stimulable phosphor layer, which is formed with the process for coating the particles of the stimulable phosphor onto the substrate, the particles of the stimulable phosphor have an approximately spherical shape, and the emitted light, which has been generated within the stimulable phosphor layer and is radiated out through protruding regions of the stimulable phosphor particles protruding at the surface of the stimulable phosphor layer, is refracted at the surfaces of the protruding regions of the stimulable phosphor particles and is converged toward the direction, which is normal to the surface of the stimulable phosphor layer. Also, in the cases of the stimulable phosphor layer, which comprises the pillar-shaped crystals of the stimulable phosphor and is formed with the vacuum evaporation process, since the top end regions of the pillar-shaped crystals of the stimulable phosphor have a protruding shape, the emitted light, which has been generated at positions deeper than the protruding regions of the pillar-shaped crystals of the stimulable phosphor and is radiated out through the protruding regions, is refracted at the surfaces of the protruding regions, i.e, the top end regions, of the pillar-shaped crystals of the stimulable phosphor and is converged toward the direction, which is normal to the surface of the stimulable phosphor layer.
In certain types of radiation image read-out apparatuses, a detection surface for the detection of the light emitted by the stimulable phosphor layer is located such that the detection surface stands facing the surface of the stimulable phosphor layer in parallel, and the light emitted by the stimulable phosphor layer is thus detected. The radiation image read-out apparatuses, wherein the detection surface is located in the orientation described above, is capable of efficiently detecting the light emitted by the stimulable phosphor layer, which has the light emission angle distribution wherein, as the light radiating angle of the emitted light becomes close to zero degree (i.e., close to the direction normal to the surface of the stimulable phosphor layer), the light intensity of the emitted light becomes high.
Also, in different types of radiation image read-out apparatuses for detecting the light emitted by the stimulable phosphor layer, the emitted light, which has been radiated out from the stimulable phosphor layer, is detected from a direction (hereinbelow referred to as the oblique direction) inclined with respect to the direction normal to the surface of the stimulable phosphor layer. In further different types of radiation image read-out apparatuses, the emitted light, which has been radiated out from the stimulable phosphor layer, is detected from the oblique direction, and the stimulating rays are irradiated to the stimulable phosphor layer from the direction normal to the surface of the stimulable phosphor layer, such that a shift in emitted light detecting position on the stimulable phosphor layer may be suppressed. (The irradiation of the stimulating rays from the direction normal to the surface of the stimulable phosphor layer will hereinbelow be referred to as the perpendicular irradiation.) (The radiation image read-out apparatus, wherein the emitted light is detected from the oblique direction, and the stimulating rays are irradiated to the stimulable phosphor layer from the direction normal to the surface of the stimulable phosphor layer, is described in, for example, patent literature 1).
In the aforesaid further different types of radiation image read-out apparatuses, a shift in emitted light detecting position on the stimulable phosphor layer is suppressed with the effects described below. FIG. 12 is an explanatory view showing how an incidence position of stimulating rays upon a radiation image storage panel varies in cases where the stimulating rays are irradiated obliquely to the radiation image storage panel. FIG. 13 is an explanatory view showing how an incidence position of stimulating rays upon a radiation image storage panel varies in cases where the stimulating rays are irradiated perpendicularly to the radiation image storage panel. Specifically, as illustrated in FIG. 12, at the time at which the stimulating rays are irradiated to the stimulable phosphor layer from an oblique direction (the irradiation of the stimulating rays from the oblique direction will hereinbelow be referred to as the oblique irradiation), and the light emitted by the stimulable phosphor layer is being detected, if the position of the stimulable phosphor layer varies vertically with respect to the stimulating rays, the problems described below will occur. More specifically, for example, if the position of the stimulable phosphor layer 1 varies downwardly with respect to stimulating rays Le as illustrated in FIG. 12, the incidence position of the stimulating rays Le upon the stimulable phosphor layer 1 will vary from an initial incidence position J1 to an incidence position J2, and a shift in emitted light detecting position on the stimulable phosphor layer 1 will thus occur. However, as illustrated in FIG. 13, in cases where the stimulating rays Le are irradiated perpendicularly to the stimulable phosphor layer 1, and the position of the stimulable phosphor layer 1 varies downwardly with respect to the stimulating rays Le, an initial incidence position J3 of the stimulating rays Le upon the stimulable phosphor layer 1 and an incidence position J4, which occurs after the position of the stimulable phosphor layer 1 has varied downwardly with respect to the stimulating rays Le, coincide with each other, and therefore a shift in emitted light detecting position on the stimulable phosphor layer 1 does not occur. Accordingly, in cases where the position of the stimulable phosphor layer 1 varies vertically with respect to the stimulating rays Le, and a radiation image read-out apparatus is employed, wherein the stimulating rays Le are irradiated perpendicularly to the stimulable phosphor layer 1, and wherein the light emitted by the stimulable phosphor layer 1 upon its exposure to the stimulating rays Le and radiated out from the stimulable phosphor layer 1 is detected with a detecting section 2, which is located obliquely, the operation for reading out the radiation image from the stimulable phosphor layer 1 is capable of being performed without being adversely affected by the vertical variation of the position of the stimulable phosphor layer 1 to the stimulating rays Le.
Patent literature 1: U.S. Pat. No. 5,055,681
However, the emitted light, which is radiated out from the stimulable phosphor layer, has the light emission angle distribution such that, as the light radiating angle becomes large, the light intensity becomes low. Therefore, in cases where the emitted light, which has been radiated out from the stimulable phosphor layer, is detected from the oblique.
The problems described above should ordinarily be taken into consideration regardless of the cases where the stimulating rays are irradiated obliquely to the stimulable phosphor layer, i.e. regardless of the angle of incidence of the stimulating rays upon the stimulable phosphor layer.